INTRODUCTORY ESSAY. 175 A combination of these three modes of division will be our usual mode of defining the localities ,of the plants. In the great majority of cases, these terms are abundantly suffi- - cient for our purposes, the range of each species being very considerable. There are, however, many instances in which it is desirable to enter into further -detail, and in such -eases we.shall ..either nfake-use of the river valleys (a very con- venient mode of indicating the-regions), or of the political subdivisions usually recognized. To these we- shall refer in the following remarks on the great geographical divisions, which correspond to the longitudinal divisions given above, with the addition of a fourth, namely, Tibet, which includes not only the-Tibetan slope of the "Himalaya,—that is to say, the ramifications which extend from its axis towards the TiBetan Brahmaputra- and Indus,—but also the mountainous country to the north of these rivers, as far as the axis of the chain of the Kouenlun. Eastern Himalaya. In this are included the states of Sikkim and Bhotan, and the districts lying to the eastward of- the latter as far as the great bend of the Brahmaputra, which we shall call collec- tively by the name of Abor. 1. ABOR. To the eastward of the Subausiri river there is'probably only one range of any considerable elevation, and the moun- tains by which the Himalaya terminates iu that direction p§r- haps nowhere attain a greater height than eight or ten thou- sand feet, while the valley of the Dihong or Brahmaputra is probably broad and open. These mountains jure inhabited by wild and suspicious tribes, who have hitherto refused all access to the interior of their- country. The climate and vegetation arc probably identical with those of the Mishmi mountains, to the eastward of the Brahmaputra, which \vill be noticed in a future page.